Early childhood development (ECD) -- cognitive, non-cognitive and physical health -- of pre-school age children is increasingly seen as critical in health and socioeconomic well-being later in life. But over 240 million children under 5y of age in developing countries do not reach their developmental potential. Advocacy for and resources devoted to ECD have increased substantially in developing countries. Yet what is known about impacts of such programs is based on few studies usually with small samples, limited outcome indicators, limited program information, small scale of operation and often problems of interpreting associations as reflecting causal effects. This project promises substantial increases in knowledge of ECD by exploiting a rich new data set with a national representative longitudinal sample of 15,000 children initially 0-5y of age and their families with linked data that permit extensive characterization of the children and their family members and the ECD programs to which they have been exposed in Chile. The Specific Aims are: (1): To enrich a public-use panel data set initiated in 2009 and to be continued at least through 2011 on 15,000 children initially 0-5y of age and their families in a nationally representative sample that is linked to other data (that include economic, health and other relevant information on all family members; child school and national test performance and ECD Center quality) with embedded experiments. (2): To describe age-gender-SES-regional gradients/inequalities in ECD program participation and quality and in ECD outcomes including cognitive development, non-cognitive development and physical health. (3): To analyze ECD program participation. (4): To analyze impacts of ECD programs, including quality and curricular focus, on child cognitive development, non-cognitive development and physical health. (5): To analyze impacts of ECD programs on mothers' time use (e.g., hours worked) and their general life satisfaction and depression. (6): To estimate cost effectiveness and cost-benefit ratios for various interventions to improve ECD to provide better tools for policy makers to make more informed choices among intervention options. (7): To simulate expected impacts of interventions to improve ECD on age-gender-SES-regional gradients/inequalities in ECD access and in cognitive development, non-cognitive development and physical health outcomes. The combination of special data features (e.g., large, nationally-representative sample; linkages to other data sources; indicators of cognitive, non-cognitive and physical development; information on ECD program quality; experiments) and methodologies not applied before to ECD in developing countries (e.g., structural models with intergenerational links in un-observables, selection regarding programs and heterogeneous impacts) promises substantial gains in knowledge about ECD. Chile is a particularly interesting context in which to study ECD because of its leadership in innovative social programs, some unique programs not present elsewhere (e.g., vouchers), rich data described above and possible implications for other middle-income as well as high-income countries.